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Full text of "Circuits"

A Touchless 3D Tracking Interface 



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A Touchless 3D Tracking 
Interface 

Written By: Steve Hobley 



TOOLS: 


PARTS: 


Solderina iron (1) 


pieces of cardboard (3) 


Spray mount or a glue stick (1) 


10KQ resistors (3) 


Wire cutter/stripper (1) 


from RadioShack. 




220KQ resistors (3) 




from RadioShack. 




Alliaator clips (3) 




from RadioShack. 




Arduino (1) 




from RadioShack. 




Shielded cabled) 




from RadioShack. 




Aluminum foil (1) 




Masking tape (1) 




Computer with Processing and Arduino 




software installed (1) 



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A Touchless 3D Tracking Interface 
SUMMARY 



In this project, we'll take a very simple idea -- the length of time it takes a capacitor to 
charge -- and make something rather amazing with it: a 3D interface that can track the 
position of your hand. The original implementation of this project comes from media artist 
Kyle McDonald . 



Step 1 — Making the sensor cube. 



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• For this project we'll need some cardboard squares (mine are approximately 12" square, 
but anything from about 8-12" is fine), alligator clips, an Arduino, shielded wire, aluminum 
foil, spray glue, and some wide tape. (3) 10KQ and (3) 220KQ resistors are also required 
(not shown). 

• Additionally, we'll need a soldering iron to connect the resistors to the cable. 

• Spray the glue onto the cardboard and smooth the foil onto one side. Work slowly and try 
to leave a small gap around the edge of the foil. It is important that the foil plates do not 
touch each other when we tape the three sides together. In a pinch you can use a glue 
stick. 

• I cut a small notch in the inside edge of the rightmost plate to make it easier to connect the 
alligator clips later on (see second photo). 

• Using tape, assemble the plates to form one half of a cube (see the third photo). 



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A Touchless 3D Tracking Interface 



Step 2 — Prepare the wires. 



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• Next we need to prepare the wiring. I bought a 6' shielded audio cable for this. It is 
important to use shielded wire so that the cable itself doesn't act as an antenna and skew 
the sensor readings. 

• Cut the wire into 3 pieces of equal length (2' or so) and strip off the outer and inner 
insulation (second photo). 

• On one end, trim off the shielding wire and connect just the inner signal wire to an alligator 
clip (third photo). 



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A Touchless 3D Tracking Interface 



Step 3 — Adding the resistors. 



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• On the other non-alligator end of the cables, twist together the 3 shield wires and solder 
them. The shield will be connected to the 5V pin on the Arduino. This will minimize the 
antenna effect of the cable on the circuit. 

• Referring to the schematic, connect the resistors to the three inner wires of the cables as 
shown and connect this to the ends of the three wires. The 220KQ resistors all connect 
between the inner wire of the cable and 5V. The 10KQ resistors will each be connected 
between the end of the cable and a pin on the Arduino. The circled area indicates that this 
wire should be shielded, with the shield connected to +5V. 

• Use a small piece of jumper wire to make the connection between the shield wires and the 
5V output pin on the Arduino, as shown in the third photo. 

• In the picture, I have used some blue heat shrink tubing to hold the three wires 
together. You could also just tape the wires together. 



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A Touchless 3D Tracking Interface 



Step 4 — Connecting the Arduino 




• Connect each of the 10KQ resistors to pins 8,9, and 10 respectively. Connect the red wire 
to the +5V pin on the Arduino. 

• Attach each of the alligator clips to a foil plate. The clips should be attached in the 
following order: pin 8=left plate (x), pin 9=bottom plate (y), pin 10=right plate (z). Make 
sure that each clip is making good electrical contact with the foil and is only touching one 
plate. 

• When troubleshooting, check the connection between the plate and the pin with a 
multimeter to make sure the alligator clip is making a good electrical contact. 



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A Touchless 3D Tracking Interface 



Step 5 — Using the software. 




• Download the Arduino sketch and Processing sketch from GitHub. If you haven't 
previously installed Processing, get it here . 

• From the Arduino IDE, upload the sketch to the Arduino. Keep the USB cable connected to 
the Arduino, then install and load the Processing sketch. 

• It is important that your computer is plugged into the wall for this to work properly. If the 
circuit isn't connected to earth ground (at least indirectly though your computer) you may 
find that your computer itself will affect the circuit. 

• The next thing to be done is calibrate the software. With the Processing sketch running, 
hold down the left mouse button. Then move your hand from the far outer diagonal corner 
(i.e., the invisible corner of the cube closest to you) to the inner corner. Don't touch the 
foil, just move your hand through the space defined by the cube. Now release the mouse 
button. The path your hand traveled gives the software a chance to detect the range of 
motion your hand will make inside the cube. 

• Now move your hand around inside of the cube. There should be a sphere on the screen 
through which you can control and move to touch all 27 cubes inside the computer model. 
Pressing tab will change the color of the cubes. 

• This is the simplest form of capacitive sensing available. In the final section, we'll 
look at some alternative solutions and options to improve the system. 







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A Touchless 3D Tracking Interface 

Step 6 — Possible improvements. 



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A Touchless 3D Tracking Interface 



r#%%*' 




• Increasing the resistor values - the 
220KQ resistors form part of the 
RC network and it's worth 
experimenting with different values. 
Typical values are 200KQ to 
500KQ. The responsiveness of the 
system will be affected by these 
values. You can also tweak the 
values in the "Normalize" tab of the 
Processing sketch to calibrate your 
particular configuration. 

• A more solid connection with 
ground. Since the Arduino is 
connected to the computer, and the 
computer is connected to the wall 
socket ground, this is generally 
good enough to provide a 
consistent reference to ground. If 
you have a house ground 
connection available you might 
want to connect this to the Arduino 
ground pin and see if that improves 
the response. 

• If you decide to connect 
anything to the house 
wiring, be very sure that it really is 
a ground. A good earth ground 
would be a water pipe. 

• The CapSense library is an 
excellent capactive sensing library 
for Arduino. The page includes 
some good information on 
configuring capacitive sensing 
circuits. 

• Terry Fritz (a well regarded Tesla 



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A Touchless 3D Tracking Interface 



Coil experimenter) put together a 
neat "vision" system for robotics 
called ThereminVision . It uses 
tuned oscillators and heterodyning 
to implement atheremin circuit. 
Definitely worth a look. 



In this guide, we introduced the concept of capacitive sensing. Nearly all sensing of this kind 
depends upon how long it takes a capacitor to charge (known as the time constant). Placing an 
object within the electric field of a capacitor will affect the capacitance value and the 
corresponding time constant. See the RC time constant entry on Wikipedia for more information. 

We hope you appreciate the cleverness of this project as much as we did and how relatively 
easy it is to get a very 21st century interface (however crude) out of some aluminum foil and a 
cardboard box! We'd love to hear of your experiences with the build (in the comment section 
below). 

last generated on 201 2-1 1 -05 05:1 6:26 PM. 



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